In semiconductor chip assembly, there are many instances in which a layer of metal or conductive material has to be deposited on a wafer. One instance involves the depositing on the wafer of soldering layers which are used to bond parts together. The solder may be placed between a photodetector and a substrate, melted in contact with the two parts, and then resolidified to create a bond between them. The solder is typically comprised of various metal components, such as a eutectic gold and tin solder. To produce high-strength and reliable bonds between the parts--as is required for semiconductor applications--it is important to achieve good uniformity in the thickness and composition of the solder plated on the wafer.
Typically, fabrication of a multi-layered solder on the wafer has been achieved by a plasma vapor deposition (PVD) process, which involves placing the wafers in a reaction chamber and evaporating gold and tin in the chamber. The PVD process is time consuming (in light of the time required to evaporate the metals), and it also is expensive. Much of the gold and tin upon evaporation becomes deposited on the chamber walls, rather than on the wafer, leading to a waste of precious metals which is costly.
Electrochemical deposition of gold and tin or other solder elements onto the wafer has been considered for avoiding the cost and inefficiencies of PVD, as electroplating uses less material than evaporation. Electrodeposition also uses less material than sputtering, another form of metal-layer deposition. However, typical electrodeposition processes are not well-suited for the handling of fragile semiconductor wafers, and they do not provide the uniform layers required for wafer applications. To illustrate, FIG. 1 shows a schematic diagram of a traditional electrodeposition apparatus. The article to be plated, i.e., the wafer 10, defines the cathode of the electrolytic cell 8, and a rod, bar or wire mesh 12, which may be fabricated with the plating metal, is the anode. The wafer 10 may be held within the electrolytic cell 8 with a bracket 14. A contact probe 16 coupled to a current source 18 touches the wafer surface on the side where plating occurs. An electrolyte (not shown) containing ions of the metal to be deposited is placed in the cell, and as current is generated in the cell between the anode and cathode, metallized ions are deposited on the cathode (i.e., the wafer).
There are a number of drawbacks with this conventional cell design as applied to the coating of semiconductor wafers. One of the challenges with electroplating wafers involves providing a good electrical contact to the wafer which has been attempted by having the probe 16 touch the wafer surface. Directly contacting the wafer, however, may cause it to break, crack, or otherwise incur damage. Placing the probe between the wafer surface and the anode (as in FIG. 1), also impedes the deposition of a uniform layer. The probe blocks the deposition of metal at the point where the probe meets the wafer, i.e., at point 10a (FIG. 1), and since the probe 16 is interposed between the cathode 10 and anode 12, it disturbs the current distribution between them. Furthermore, if the probe 16 is not completely sealed from the electrolyte, metal ions may be deposited on the probe, rather than on the wafer. Each of these features reduces the efficiency of the process for electroplating wafers and leads to the production of non-uniform, non-reproducible films.
Thus, it would be advantageous to provide an apparatus for coating wafers via electrochemical deposition which is efficient, produces uniform films, and accommodates the fragile nature of wafers. The invention provides such an apparatus. Further advantages may appear more fully upon considering the description below.